Carbon nanotubes (CNT) are of substantial interest for numerous potential applications due to their unique electrical, thermal, and mechanical properties. For example, certain nanotubes are believed to have strength on the order of 100 s of GPa, or more than 100 times stronger than steel. They also have unique electrical properties that make them attractive for use in solar cells, capacitors, batteries and other energy storage devices, as conductive coatings, in gas sensors, etc.
Impurities in CNT materials appear in multiple forms and are often introduced during the synthesis of CNT. In a typical manufacturing process called alcohol catalytic chemical vapor deposition (ACCVD), evaporated methanol or ethanol vapors come in contact with catalyst particles such as nickel or iron, embedded on magnesium oxide or silica as catalyst support, at high temperatures inside a furnace. At such conditions, ethanol or methanol molecules break down, and CNT start growing around the catalyst. However, this process also results in the generation of amorphous carbon, which can be located randomly on the outer surfaces of CNT. Amorphous carbon is the most common impurity and the hardest to remove, due to bonding on certain carbon atoms. Other types of impurities include catalyst residue such as iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), etc., and catalyst support materials such as magnesium oxide (MgO), aluminum oxide (Al2O3), and silicon dioxide (silica, SiO2).
Amorphous carbon and inorganic impurities in CNT materials are detrimental to the electrical, thermal, and mechanical properties of the material. The presence of impurities can affect the properties to an extent that renders the CNT material unsuitable for many applications. For this reason, significant effort has been undertaken to produce purified CNT.
A common method of evaluating CNT for the presence of amorphous carbon is by visual observation at high magnification (greater than about 20,000×). This can be accomplished using commercially available instruments such as a scanning electron microscope (SEM) or transmission electron microscope (TEM). The visual appearance of amorphous carbon under high magnification is quite distinct from that of CNT and CNT bundles, and good qualitative evaluations of amorphous carbon content in a CNT sample can be achieved by this method.
Evaluation of inorganic impurity contact in a CNT material can be accomplished by a variety of means to different levels of accuracy and precision. Common methods include Energy Dispersive X-Ray Spectroscopy, which is usually conducted in concert with SEM or TEM, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The former is a semi-quantitative method that is most useful for identifying, rather than quantifying, the impurity elements present in a CNT sample. The latter can be used to determine precise amounts of those elements in the material.
Another technique useful for evaluating the quality of CNT, i.e., the concentration of structural defects and amorphous carbon impurities included therein, is by measuring the intensity ratio of two characteristic Raman spectral peaks, called the G/D ratio. The G-band is a tangential shear mode of carbon atoms that corresponds to the stretching mode in the graphite plane. The D-band is a longitudinal optical (LO) phonon and is known as the disordered or defect mode, as it is a typical sign for defective graphitic structures in CNT. The comparison of the ratios of these two peaks' intensities gives a measure of the quality of the CNT samples. Generally, the G/D ratio is used to quantify the structural quality of carbon nanotubes. Thus, CNT having a higher G/D indicate a lower amount of defects and a higher level of quality.
A G/D ratio is typically determined using a Raman spectroscopy technique. Any of various commercially available instruments may be used to measure the G and D band intensities and to calculate the G/D ratio. One example of such equipment is available from HORIBA Jobin Yvon Inc., Edison, N.J., under the model name LabRAM ARAMIS.
The G/D ratio usually changes after a purification is applied to a sample of CNT. When purified of amorphous carbon, the G/D ratio of the purified CNT is typically greater than the G/D ratio of the starting CNT, indicating that the purified CNT has fewer structural defects and/or carbonaceous impurities with different carbon bond types than that of CNT. For removal of inorganic impurities, prior art methods, such as reacting in highly concentrated acids at elevated temperatures, typically result in significant decrease in G/D ratio, indicating that the purification process imparted damage to the CNT structure.
Various methods of removing amorphous carbon and inorganic impurities are known in the literature, including thermal oxidation, various solution treatments, and various gas treatments. However, existing methods tend to damage CNT (as mentioned above), cause significant loss of CNT, or result in only partial purification. Known commercial methods typically entail treatment of CNT with concentrated acid, such as nitric acid, often at elevated temperatures, followed by a slow heat treatment. Although this protocol has been proven to reduce both amorphous carbon and inorganic content, it is unsafe, and a substantial amount of contamination can still remain on the surface. Furthermore, treatment with concentrated acids is somewhat counterproductive, as it also introduces structural defects while removing superficial ones.
Therefore, there exists a need for an efficient and safe process for preparing purified CNT; the method should efficiently remove all or nearly all amorphous carbon and inorganic impurities without damaging or destroying the CNT.